This paper proposes a definition, scope and topics of construction informatics--a discipline also known as `construction IT' or `communication and information technologies in construction'. It presents its ontology that, together with methodology, epistemology and axiology, constitutes a formal definition of a scientific field. In philosophy, ontology defines `what exists'. In the case of a scientific field--what exists for a discipline, what is its area of discourse. The given ontology of construction informatics has a shape of a hierarchy. On the top-level, it distinguishes between (1) core themes and (2) support themes. Core themes create knowledge related either to (a) information processing activities, (b) communication/coordination activities or about (c) common infrastructures. The support themes address the knowledge transfer process and include research needs, transfer, deployment and impact of research. The ontology can be used to map a research area, to desig...